1. Field of the Invention
The present invention relates to a collimator used for an X-ray CT (Computer Tomography) apparatus, a method of manufacturing the collimator, and an X-ray CT apparatus having the collimator.
2. Description of the Related Art
As is well known, an X-ray CT apparatus is designed to obtain an image (tomographic image) by calculating (reconstructing) the X-ray absorptance of tissue such as an organ as an index called a CT value with reference to the X-ray absorptance of water on the basis of the amount of X-rays absorbed by a subject.
This X ray CT apparatus is provided with a collimator on, for example, the X ray incident side of an X ray detector to reshape the shape of an X ray beam striking each X ray detection element and to remove scattered X rays. FIG. 1A shows an example of the arrangement of a conventional collimator having an integral structure (to be referred to as an “integral collimator” hereinafter). As shown in FIG. 1A, the integral collimator has upper and lower arcuate supports arranged side by side in the slice direction along the body axis of a subject. Pairs of upper and lower grooves are formed in the upper and lower supports so as to allow the insertion of collimator plates therein such that the respective plates face an X ray focal point (which is assumed to be the emission point of an X ray source). Flattened collimator plates are inserted in these grooves. An adhesive is then applied to the portions of the plates which are inserted in the grooves and is cured, thereby forming a collimator as an integral structure. The collimator plates are supported by the grooves of the upper and lower supports, and reshape incident X rays without degrading the warpage of each plate owing to its rigidity.
Assume that such an integral collimator comprises, for example, collimator plates each having a length of less than 100 mm in the slide direction in an X-ray CT apparatus. In this case, if flattening processing is performed for each collimator plate in advance, a collimator with little warpage on the 20 μm order can be formed with only the rigidity of each collimator plate.
The recent trend is to develop X-ray CT apparatuses with wider detection ranges in the slice direction. In an X-ray CT apparatus having 256 rows of multi-slice detectors which has currently been developed, the detection range in the slice direction is assumed to be about four times that in existing X-ray CT apparatuses. For this reason, according to the arrangement of a conventional integral collimator, it is difficult to maintain the flatness and warpage of each collimator plate with only the rigidity of each collimator plate. As a consequence, when each detector (detector unit) is to be mounted, alignment cannot be performed, and the solid angle of an X-ray beam striking each X-ray detection element cannot be properly limited, resulting in failure to acquire an appropriate tomographic image.
In order to solve this problem, for example, as shown in FIG. 1B, there has been proposed a collimator having a module structure (to be referred to as a “module type collimator” hereinafter) which covers about 20 channels of a detector. As shown in FIG. 1B, a plurality of such module type collimators are arranged to cover the entire detection surface of the X-ray detector along the channel direction. The module type collimator has front and rear supports, in each of which grooves in which collimator plates are to be inserted are formed. These grooves are formed in the front and rear supports at different pitches because collimator plates need to face the X-ray focal point. By inserting collimator plates in the pairs of grooves in the front and rear supports, the collimator plates form an arrangement widening toward the end. As a result, all the collimator plates are formed to face the X-ray focal point.
Such a module type collimator is assembled while adjusting the squareness with respect to an end face of each support or the reference surface of the central plate, thereby forming a module type collimator set at a correct position opposing the X-ray focal point. It has been confirmed that in even a region where the detection range of each collimator plate in the slice direction is about 200 mm or more and hence it is difficult to flatten collimator plates, warpage is corrected by inserting plates in the grooves formed in the front and rear supports in the slice direction, and a collimator which maintains flatness as in existing collimators can be formed. As a consequence, alignment with the detector can be done.
In the above module type collimator as well, for example, the following problem arises.
In the module type collimator, factors that unstabilize the mount surface of the detector module on which the collimator is mounted cannot be eliminated. Even if, for example, a dust particle on the 10 μm order exists on the mount surface, the X-ray focal point at the position about 1 m ahead of the dust particle is enlarged and shifted. Therefore, steps are produced in continuity that connects the X-ray focal point at the joint portions between the module type collimators. As a consequence, when the polar response characteristic, i.e., the X-ray foal point, shifts over time, an impermissible unbalance amount, which cannot be neglected, is produced in variation components of shadow on the detector, resulting in the production of artifacts in an image.